Remove usage of Polymer internal helpers from Lit based components

[web-padawan]

Describe your motivation

While some components already provide Lit based versions, we still have a few Polymer imports like afterNextRender:

https://github.com/vaadin/web-components/blob/68a5c78fff76f213fae11b807e42b0841d52f232/packages/overlay/src/vaadin-overlay-mixin.js#L6

https://github.com/vaadin/web-components/blob/a57bae74645429380e44f281915c57b1df33def3/packages/date-picker/src/vaadin-date-picker-overlay-content-mixin.js#L6-L7

Describe the solution you'd like

Replace these with custom alternatives, e.g. afterNextRender essentially is requestAnimationFrame + setTimeout.